1×N fanout waveguide photodetector

ABSTRACT

A 1×N fanout waveguide detector is disclosed. The detector includes a multiple-mode interference (MMI) cavity with input and output ends. A single-mode waveguide is optically coupled to the input end of the MMI cavity so that the optical power in the guided mode is distributed over N modes. The MMI cavity forms N interference nodes at or near its output end. N waveguide detectors are optically coupled to the output end at or near the N interference nodes. The waveguide detectors each have a waveguide that is evanescently coupled to an intrinsic region of a PIN detector. The width of the detector waveguide core, which can be sub-micron, defines the carrier collection distance between the electrodes of the PIN detector. Further, the length of the detector waveguide can be selected to maximize optical absorption to provide optimum quantum efficiency. The waveguide detectors are connected in parallel to provide a high-output photocurrent.

RELATED APPLICATION(S)

This application is a Divisional of U.S. application Ser. No. 10/008,922filed Dec. 7, 2001, now U.S. Pat. No. 6,856,733, which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention pertains to photodetectors, and in particular towaveguide-based high-speed photodetectors.

BACKGROUND OF THE INVENTION

There are many lightwave applications, such as opticaltelecommunications and chip interconnects, that involve transmittingoptical signals and converting them to electrical signals at high datarates. The systems for performing such transmission and conversionusually require a photodetector compatible with the speed and bandwidthof the optical signal. The typical photodetectors are PIN(p-type/intrinsic semiconductor material/n-type) semiconductordetectors.

To date, it has been a challenge to make a Si-based semiconductor PINphotodetector with a bandwidth of 10 GHz or greater. Conventionaldiscrete PIN Si detectors operate at speeds of 2 GHz or less because oftheir relatively low absorption coefficient and low carrier collectionefficiency. The best Si detector known today is the interdigitatedlateral trench device (LTD), which operates at speeds of up to 6.5 GHzdue to improved absorption by the trench structure.

It is well known that excess optical power density in PIN photodetectorscauses detector speed degradation. This is especially true forwaveguide-based PIN photodetectors because light is coupled in to asmall region about the size of the waveguide. As a result, the detectormay not be able to operate at high photocurrent where high-speedoperation may require a high current. Where such a system employsevanescent coupling to the intrinsic region of the PIN detector, theintrinsic region can be expanded to dilute the optical power, which inturn prevents the creation of excess carriers. However, the lightdistribution in the expanded waveguide region (i.e., the waveguide plusthe intrinsic region) is not uniform so that the detector electrodesneed to be made relative large to ensure adequate detection of thephoton-generated carriers. Unfortunately, the relatively large electrodearea results in a relatively high detector capacitance, which reducesdetector speed. Further, the non-uniform distribution of light in theexpanded waveguide region can result in high optical fields, whichgenerate local excess photon-generated carriers. This reduces thedetector speed when the excess carriers have to diffuse out of the localexcess carrier area to be collected by the electrodes.

Further, photon-generated carriers formed in the intrinsic region of aPIN detector may collected either via electrodes in the top or bottom ofthe detector as discussed above, or by metal-semiconductor-metal (MSM)interdigitated electrodes on the surface of the intrinsic region. In thefirst design, the carrier collection distance is set by the requiredminimum detector thickness for efficient light absorption due toevanescent coupling with the waveguide. This thickness, however, limitsthe detector speed. In the latter design, photon-generated carriers alsohave to travel to the interdigitated electrodes across the detector, sothat the detector thickness also limits the detector speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the waveguide photodetector of the presentinvention, with the guided modes illustrated schematically as lightrays;

FIG. 2 is a cross-sectional view of the waveguide photodetector of FIG.1 showing the core and cladding for the input waveguide, the MMI cavityand one of the detector waveguides, with the guided modes illustrated aselectromagnetic waves;

FIG. 3 is a perspective end view of one of the waveguide detectors,showing the relationship between the waveguide portion of the detector,the underlying intrinsic layer, and the p+ and n+ electrodes surroundingthe intrinsic layer, along with the conformal cladding layer (dashedline);

FIG. 4 is side view of a substrate with a silicon-on-insulator (SOI)structure formed thereon;

FIG. 5 is a top-down perspective view of the SOI structure of FIG. 4,with islands are formed from a portion of the silicon layer;

FIG. 6A is a side view of the structure of FIG. 5, with insulationregions formed between the silicon islands;

FIG. 6B is the same side view as FIG. 6A, further including an optionalthin layer of oxide atop the structure;

FIG. 7A is a side view of the structure of FIG. 6A, with a waveguidelayer formed atop the structure;

FIG. 7B is a top-down perspective view of the structure of FIG. 7A, butwith the waveguide layer processed to create the core regions for theinput waveguide, the MMI cavity, and the N detector waveguides;

FIG. 8 is top-down perspective view of the structure of FIG. 7, with n+and p+ electrodes formed in portions of the silicon island adjacent eachwaveguide detector core to form PIN detectors beneath each waveguidedetector, and also showing the conformal cladding layer formed over thestructure;

FIG. 9A is two-dimensional simulation of the optical power distributionin an example waveguide photodetector of the present invention having a1×8 fanout MMI coupled to a single-mode input waveguide with a siliconcore having a width of 0.25 microns surrounded by a SiO₂ cladding, theguided light having a wavelength of 1.3 microns;

FIG. 9B is two-dimensional simulation of the optical power distributionin an example of a waveguide photodetector of the present inventionhaving a 1×14 fanout MMI coupled to a single-mode waveguide with a Si₃N₄core having a width of 0.6 microns surrounded by a SiO₂ cladding, theguided light having a wavelength of 850 nm;

FIG. 10 is a plot of the normalized optical power distribution in adetector waveguide (solid line) along with a plot of the optical powerin the intrinsic region below the waveguide (dashed line) illustratingthe power transfer from the detector waveguide to the intrinsic regionof the PIN detector as a function of the length of the intrinsic region;

FIG. 11 is a schematic diagram of an optoelectronic system that includesthe waveguide photodetector of the present invention;

FIG. 12 is a schematic diagram of an on-board or on-chip optoelectroniccommunication system incorporating the waveguide photodetector of thepresent invention as a more detailed example of the generalizedoptoelectronic system of FIG. 11; and

FIG. 13 is a schematic diagram of an optoelectronic clocking circuitincorporating the waveguide photodetector of the present invention, as amore detailed example of the generalized optoelectronic system of FIG.11.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the embodiments of theinvention, reference is made to the accompanying drawings that form apart hereof, and in which is shown by way of illustration specificembodiments in which the invention may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that changes may be made withoutdeparting from the scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims.

With reference to FIGS. 1 and 2, there is shown an integratedwaveguide-based photodetector system 10 comprising an input waveguide 20having a core 22 and a cladding 24. In an example embodiment, thewaveguide is designed to support a single waveguide mode 28, asillustrated schematically in FIG. 1 as a light ray and in FIG. 2 as anelectromagnetic wave. The waveguide may be designed to support more thana single waveguide mode.

Input waveguide 20 also has an input end 34, an output end 36, and isoptically coupled to a multi-mode interference (MMI) cavity 40 at theoutput end. The input waveguide may comprise any semiconductor ordielectric material transparent to the wavelenth of light beingdetected. In an example embodiment, the material making up core 22 ispreferably high index, and the material making up cladding 24 ispreferably low index. For example, the waveguide may comprise Si₃N₄ fortransmission of light having a wavelength of 850 nm or intrinsic siliconfor wavelengths of 1 micron or greater. Input waveguide cladding 24 maybe, for example, SiO₂, which has a relatively low refractive index(about 1.5) as compared to that of Si₃N₄ (about 3.5) at near-infra-redand infra-red wavelengths. Use of a high-index core and a low-indexcladding allows for the input waveguide to have a relatively small(i.e., sub-micron) thickness T1 (X-dimension) and width W1(Y-dimension).

MMI cavity 40 has an input end 44 to which output end 36 of inputwaveguide 20 is optically coupled. MMI cavity 40 also has an output end46 opposite input end 44, and a length L2. MMI cavity 40 is formed froma semiconductor or dielectric material transparent to the wavelength oflight being detected. For the sake of convenience, MMI cavity 40preferably comprises the same material as input waveguide 20 so that theinput waveguide and the MMI cavity can be formed as an integratedstructure. In an example embodiment, the thickness T2 of MMI cavity 40is the same as the thickness T1 of waveguide 20, while the width(Y-dimension) W2 of the MMI cavity is greater than the width W1 of thewaveguide.

By way of example, input waveguide 20 may have a width W1 in the rangefrom about 0.1 to about 0.5 microns, while MMI cavity 40 may have awidth W2 in the range from about 5 microns to 10 microns. MMI cavity 40includes includes a core 52 and a cladding 54 surrounding the core. Inan example embodiment, core 52 comprises the same material as core 22and cladding 54 comprises the same material as cladding 24. Futher in anexample embodiment, cores 22 and 52 are contiguous and claddings 24 and54 are contiguous.

The length L2 of MMI cavity 40 is designed so that the incoming singlewaveguide mode 28 from waveguide 20 spreads (i.e., “fans out”) in theY-direction into N multiple waveguide modes 60 within the MMI cavity.Each mode 60 carries a corresponding fraction of the input energy ofwaveguide mode 28. In FIG. 1, multiple waveguide modes 60 areschematically represented as light rays. MMI cavity 40 serves todisperse the optical power density of single waveguide mode 28 by afactor of N. Further, the length L2 of MMI cavity 40 is designed so thatinterference nodes 66 arising from the constructive interference of theN waveguide modes (i.e., the intersection of the light rays) are locatedat or near cavity output end 46.

System 10 further includes an array 80 of N (N≧2) waveguide detectors 82optically coupled to MMI cavity 40 at output end 44 at or nearinterference nodes 66. Each waveguide detector 82 includes a waveguide100 with a core 102, a cladding 104, a lower surface 106, a core widthW_(C) and an overall width (core plus cladding) of W_(N). Waveguide 100preferably supports a single guided mode 106. Widths W_(C) and W_(N) canvary between waveguides but is preferably the same for each waveguidefor the sake of convenience. Waveguide detectors 82 further include p+and n+ electrodes 110 and 112 arranged on opposite sides of waveguide100 below the plane defined by waveguide lower surface 106.

FIG. 3 is a perspective close-up endview of one of waveguide detectors82. Electrodes 110 and 112 are separated by an intrinsic region 120residing directly underneath waveguide lower surface 106, which isoptically coupled to waveguide 100. Intrinsic region 120 therefor has awidth equal to or substantially equal to the core width W_(C) ofwaveguide 100. In an example embodiment, intrinsic region 120 is formedunder waveguide 100 in combination with the self-aligned formation of n+and p+ electrodes 110 and 112, formed with respect to waveguide core102. Intrinsic region 120 is made from a semiconductor material, and inexample embodiments comprises either silicon or germanium.

Thus, each waveguide detector 82 has a PIN configuration with thephoto-generated carrier collection distance within intrinsic region 120equal to or substantially equal to core width W_(C) of waveguide 100. Asmentioned above, it is preferable that waveguides 100 be single mode sothat the core width W_(C) is as small as possible. The core width W_(C)can be made sub-micron by using a high-index contrast between core 102and cladding 104. In an example embodiment, core 102 includes ahigh-index integrated-circuit (IC) compatible material, such as Si₃N₄surrounded by cladding 104 that includes a low index dielectric such assilicon dioxide. This allows for very fast detector speeds, e.g.,greater than about 10 GHz when detectors 82 have an intrinsic regioncomprising silicon, and greater than about 40 GHz when detectors 82 havean intrinsic region comprising germanium.

The difference in detector speeds between silicon and germanium isrelated to the higher absorption coefficient and carrier mobility ofgermanium, which is 4× faster than that of silicon. An advantage ofusing germanium in forming intrinsic region 120 is that the length L3 ofthe intrinsic region can be made short (e.g., about 5 microns) ascompared with the length associated with silicon (e.g., about 80 micronslong). Thus, forming intrinsic region 120 from germanium provides for acompact waveguide photodetector system 10 with smaller capacitance.

Each waveguide detector 82 has a longitudinal configuration where lightis transferred to and absorbed by intrinsic region 120 as it propagatesdown detector waveguide 100 (i.e., the Z direction). This allows eachwaveguide detector 82 to have high total quantum efficiency (e.g.,greater than about 80%), since the length L3 of intrinsic region 120 canbe tailored to provide the optimum absorption efficiency. The length L3of intrinsic region 120 (also called the “absorption length”) hasminimal impact on detector speed since the photon carrier collectiondirection is the Y-direction, which is perpendicular to the lightpropagation and absorption direction, which is the Z-direction.

Further, photon-generated carriers are formed mostly in intrinsic region120 rather than under n+ electrode 110 and p+ electrode 112 becausethere is no direct light path to the electrodes. Thus, fewer slow-driftcarriers, which reduce the speed of the detector, are generated.

In waveguide detector system 10, the screening effect of high-densityspace-charge fields due to high-density photon-generated carriers byhigh optical power density is mitigated by dividing the incoming powerdensity by a factor of N and then coupling the diluted power into the Nseparate waveguide detectors 82. This allows for fast detection speeds.Further, detectors 82 can be arranged to operate in parallel so that ahigh-output photocurrent (e.g., greater than about 100 microamperes) isgenerated. The amount of output photocurrent depends on the number ofparallel waveguide detectors used. In example embodiment, 10 parallelwaveguide detectors are used to generate an output photocurrent of about100 microamperes. The use of multiple parallel waveguide detectors 82provides for a minimum electrode area and thus a minimum capacitance andresistance, further increasing the speed of waveguide photodetectorsystem 10.

Method of Fabrication (FIGS. 4–10)

With reference now to FIG. 4, the method of fabricating an integratedwaveguide-based photodetector system 10 as described above begins withproviding a substrate 200. Substrate 200 is preferably formed from anIC-compatible material, such as a semiconductor material such assilicon, or saphire. For the sake of discussion, it is presumed belowthat substrate 200 is silicon.

Atop silicon substrate 200 is formed a insulating layer 206, such assilicon dioxide or other dielectric. Insulating layer 206 serves tooptically and electrically isolate substrate 200 from system 10 to beformed thereon. Thus, in an example embodiment, insulating layer 206 mayhave a thickness, for example, of about 1 to about 3 microns.

Atop insulating layer 206 is formed a semiconductor layer 210, which canbe Si, Ge, Ge_(x)Si_(1-x), or Ge on Si. For the sake of discussion, itis assumed layer 210 is Si, thereby forming a silicon-on-insulator (SOI)structure. Silicon layer 210 may have a thickness, for example, of 0.25to 0.5 microns.

In FIG. 5, silicon layer 210 is lithographically processed using wellknown techniques (e.g., coating with a layer of photoresist,photolithographically exposing the resist with a pattern, developing theresist and then etching the resist) to define silicon islands 220 over aportion of insulating layer 206.

In FIG. 6A, an oxide layer 222 (e.g., SiO₂) is deposited over thestructure of FIG. 5 and is polished, e.g., via a chemical-mechanicalpolish (CMP) process, until the top of islands 220 are exposed. Thisresults in silicon islands 220 being insulated from each other byportions of oxide layer 222.

In an example embodiment, the surface of silicon islands 220 may beoptionally processed using a standard gate oxide cycle, which includesbuff oxidation to remove CMP-induced damage, pre-clean, gate oxidation,and a passivation anneal. Further, with reference to FIG. 6B, in anotherexample embodiment, a stress release layer 230 of oxide of about 100 to200 Angstroms thick is optionally formed atop silicon islands 220 tofacilitate the next step in the process. The stress release layer mayalso serve as cladding 104 formed between intrinsic region 120 and core104 of waveguide 100 (FIG. 2).

In FIG. 7A, a waveguide layer 250 is formed atop the structure of FIG.6A (or alternatively, atop the structure of FIG. 6B). In one exampleembodiment, waveguide layer 250 comprises one of Si, Ge, Ge on Si,Ge_(x)Si_(1-x), SiO_(x)N_(y) and Si₃N₄. Waveguide layer 250 has athickness designed to support a given number of waveguide modes at agiven wavelength when surrounded by a cladding layer of a givenmaterial. Thus, in an example embodiment, the thickness of waveguidelayer 250 can range from the sub-micron (e.g., from about 0.1 micron) toseveral microns.

In FIG. 7B, waveguide layer 250 is lithographically processed to definecore 22 of waveguide 20, core 52 of MMI cavity 40 and cores 102 ofwaveguides 100, with cores 102 aligned to silicon islands 220. Each core102 does not cover the entire silicon island 220 so that portions 260and 262 of each island on either side of the core are exposed.

Exposed portions 260 are n+ doped and exposed portions 262 are p+dopedto an form n+ electrode 110 and a p+ electrode 112 adjacent each core102. In an example embodiment, doping is achieved by ion implantation ina manner that makes n+ electrode 110 and p+ electrode self-aligned withcore 102.

In FIG. 8, a cladding layer 280 is conformally deposited atop theremaining waveguide layer 250 to complete the formation of inputwaveguide 20, MMI cavity 40 and detector waveguides 100. Cladding layer280 may be any dielectric, such as silicon dioxide or polyimide, so longas the index of the cladding layer is less than that of waveguide layer250 for the particular operating wavelength.

The undoped silicon region remaining beneath each waveguide core 102becomes intrinsic region 120. The width of intrinsic layer 120 issubstantially equal to the width W_(C) of core 102 and defines thecarrier collection distance. All the n+ electrodes 110 and p+ electrodes112 are preferably connected in parallel so that detectors 82 form array80 of N parallel detectors capable of generating a high photocurrentcurrent.

With reference to FIGS. 9A and 9B, there is shown two-dimensionalsimulations of the distribution of optical power in two examplewaveguide photodetector systems 10 according to the present invention.The simulations were performed using a commercial software packagecalled the rSoft BPM simulator, available from www.rsoftinc.com.

In FIG. 9A, core 22 of waveguide 20 is modeled based on a silicon coreof width 0.25 microns surrounded by an SiO₂ cladding, thereby providingan index differential between the core and cladding of about 1.5. Theguided light has a wavelength of 1.3 microns. In the present invention,the dimensions of MMI cavity needed to achieve a given value of N forthe fanout can be readily determined by similation. MMI cavity 40 ofFIG. 9A has a length L2 of approximately 30 microns and a width W2 ofapproximately 10 microns and supports a fanout of N=8. The total opticalpower preserved within the 1×8 fanout is calculated at about 81%. Thesimulation also shows interference nodes 66 at which waveguide detectors82 are located.

In FIG. 9B, core 22 of waveguide 20 is modeled based on Si₃N₄ core witha width of 0.6 microns surrounded by an SiO₂ cladding. The guided lighthas a wavelength of 0.85 microns. MMI cavity 40 of FIG. 9B has a lengthL2 of approximately 100 microns and a width W2 of approximately 25microns and supports a fanout of N=14. The total optical power preservedwithin the 1×14 fanout is calculated at about 84%.

In FIG. 10, there is shown a three-dimensional simulation of the opticalpower distribution in a Si₃N₄ detector waveguide 100 having a core widthW_(C) of 0.4 microns. The wavelength of the guided light is 850 nm, andintrinsic region 120 beneath the waveguide is silicon with dimensions 1micron×1.2 microns×75 microns. The capacitance of the waveguide detectoris about 10fF. From the plot, it can be seen how the optical power inthe waveguide (solid line) oscillates and decays as the lightpropagating in waveguide 100 is evanescently coupled to and absorbed byunderlying intrinsic region 120. Likewise, the optical power inintrinsic region 120 (dashed line) also oscillates in synchrony (but 90degrees out of phase) with the waveguide optical power, and decays asthe light is absorbed in the intrinsic region. The absorbed lightresults in the generation of photon-generated carriers, resulting in acurrent detected by surrounding electrodes 110 and 112. The couplingefficiency is calculated to be in excess of about 90%. Quantumefficiency can be optimized by selecting the appropriate length L3 ofintrinsic region 120. The longer L3 is, the higher the quantumefficiency. However, the length L3 is limited to minimize thecapacitance. The carrier collection speed is optimized by selecting thesmallest possible cross-section core width W_(C) for waveguide 100.

Optoelectronic Systems

With reference to FIG. 11, there is shown a generalized optoelectronicsystem 400 that includes the waveguide photodetector 10 of the presentinvention. Optoelectronic system 400 includes an optical oroptoelectronic input device 410 optically coupled to input waveguide 20of waveguide photodetector 10. Input device 410 may include, forexample, a laser diode or a vertical cavity surface emitting laser(VCSEL). Optoelectronic system 400 further includes an electronic oroptoelectronic output device 420 electrically coupled to waveguidedetectors 82 to receive a photocurrent electrical signal 424 generatedby waveguide photodetector 10. Optionally included in optoelectronicsystem 400 is a second output device operatively coupled to the firstoutput device for receiving an electrical signal 444 from the firstoutput device and further processing the signal.

In operation, input device 410 generates an optical signal (i.e., light)445 and inputs the optical signal to input waveguide 20. Optical signal445 propagates down input waveguide 20 as guided mode 28. As discussedabove, guided mode 28 is fanned out (dispersed) into N guided modes 60by MMI cavity 40, which formes N interference nodes 66 (FIG. 1).Waveguide detectors 82 detect the light at interference nodes 66 andconvert the light into a photocurrent electrical signal 424, which isoutputted to and received by output device 420.

Optoelectronic system 400 represents a large number of possible systems.By way of example, with reference to FIG. 12, optoelectronic system 400represents an on-board or on-chip communication system, wherein inputdevice 410 constitutes an optical transmitter, and waveguidephotodetector 10 and output device 420 together constitute an opticalreceiver 446. Input device/optical transmitter 410 includes driverelectronics 460 (e.g., a transimpedance amplifer) electrically connectedto a laser diode 466, and a waveguide 468. Waveguide 468 is opticallycoupled to the laser diode at one end and to waveguide 20 of inputdevice/optical transmitter 410 at its other end. In an exampleembodiment, waveguide 468 is single mode. In a further exampleembodiment, output device 420 includes pre-amplifier electronics 470.

In operation, a plurality of electronic signals 474 are multiplexed by atime-division multiplexer 476 residing on a first chip or board 480 toform a multiplexed electrical signal 482. Multiplexed electrical signal482 is passed to input device/optical transmitter 410. Inputdevice/optical transmitter 410 receives multiplexed electrical signal482 and converts it to optical signal 445, which is inputted towaveguide 468. Optical signal 445 is transmitted through waveguide 468as a guided mode and passes to waveguide 20, where it continues topropagate as waveguide mode 28. As described above in connection withFIG. 1, waveguide mode 28 enters MMI cavity 40 and is fanned out(dispersed) by MMI cavity 40 and detected by waveguide detector array 80to produce photocurrent electrical signal 424.

Photocurrent electrical signal 424 is outputted to and received bypre-amplifier electronics 470 in output device 420. Signal 424 isprocesses by pre-amplifier electronics 470 to form an electronic signal444, which is passed to a time-division demultiplexer 490 as part of asecond chip or board 494 constituting second output device 440 (FIG.11). Time-division demultiplexer 490 receives electronic signal 444 andforms demultiplexed electronic signals 496 to be processed by otherelectronic elements (not shown) on second chip or board 494.

In another example embodiment, optoelectronic system 400 constitutes atelecommunication/data communication system similar in design to theon-chip or on-board system described above, with waveguide 490 being alength of optical fiber (single mode or multimode), and includinganalogous multiplexing and demultiplexing processing elements andprocesses.

In yet another example embodiment, optoelectronic system 400 can be usedto form a optoelectronic clocking source. An optoelectronic clockingsource utilizing the waveguide photodetector of the present inventioncan performing high-speed clocking, since the large amounts of opticalpower and the corresponding high photocurrent required can be handled bythe waveguide photodetector of the present invention.

With reference to FIG. 13, there is shown an example optical clockingsource system 520 that includes optical/optoelectronic input device 410coupled to an optical edge tree 530. Optical edge tree 530 comprises amain waveguide 536 from which extends a number of equal length waveguidebranches 540, each capable of supporting a portion of optical signal445. Each waveguide branch 530 is equal in length and is connected tothe input waveguide 20 of a waveguide photodetector 10.

The output of each waveguide photodetector 10 is connected to anelectrical edge tree 560 having electrical (e.g., conducting) branches570 of equal length that support a portion of photocurrent electricalsignal 424. Each electrical branch 570 of electrical edge tree 560 isconnected to an output device 420, such as a transimpedence amplifier(FIG. 11). The equal lengths of the waveguide branches and the equallengths of the electrical brances provide for the equal timing of theoptical and electrical signals necessary in a clocking circuit. Asoptical waveguide branches can be longer than electrical branches whenforming clocking circuits, the clocking signal source (e.g., inputdevice 410) can be located relatively far away (e.g., off-chip) in anoptoelectronic clocking circuit as compared to an all-electricalclocking circuit.

While the present invention has been described in connection withpreferred embodiments, it will be understood that it is not so limited.On the contrary, it is intended to cover all alternatives, modificationsand equivalents as may be included within the spirit and scope of theinvention as defined in the appended claims.

1. An system comprising: a waveguide photodetector including: a multiplemode interference (MMI) cavity having an input end and an output end; aninput waveguide optically coupled to the MMI cavity at the input end; anarray of detector waveguides optically coupled to the MMI cavity at theoutput end and each optically coupled to an intrinsic region havingfirst and second electrodes on opposite sides of the intrinsic region todetect photon-generated carriers formed in the intrinsic region andoutput a photocurrent; an input device optically coupled to the inputwaveguide that generates an optical signal and inputs the optical signalinto the input waveguide; and an output device to receive thephotocurrent.
 2. The system of claim 1, wherein each detector waveguideincludes a core, a cladding, and a lower surface and an intrinsic regiondisposed underneath the lower surface and surrounded by the first andthe second electrodes, wherein the core is longitudinally aligned alongthe intrinsic region and optically coupled to the intrinsic region. 3.The system of claim 1, wherein each detector waveguide has a core with acore width, and wherein each intrinsic region has a carrier collectiondistance substantially equal to the core width.
 4. The system of claim1, wherein the MMI cavity has a length such that interference nodes arelocated at the output end such that each interference node issubstantially at an entrance to each detector waveguide.
 5. The systemof claim 1, wherein each detector waveguide is a single mode waveguide.6. The system of claim 1, wherein each detector waveguide has a quantumefficiency greater than about 80%.
 7. The system of claim 1, whereineach intrinsic region includes germanium.
 8. The system of claim 1,wherein the input device includes a laser diode or a vertical cavitysurface emitting laser.
 9. The system of claim 1, wherein the outputdevice includes a transimpedance amplifier.
 10. A system comprising: anoptical transmitter having drive electronics; a coupling waveguide; andan optical receiver optically coupled to the optical transmitter by thecoupling waveguide, the optical receiver having a waveguidephotodetector and an output device to receive a photocurrent from thewaveguide photodetector, the waveguide detector including: a multiplemode interference (MMI cavity having an input end and an output end; aninput waveguide optically coupled to the MMI cavity at the input end,the input waveguide coupled to the coupling waveguide; an array ofdetector waveguides optically coupled to the MMI cavity at the outputend and each optically coupled to an intrinsic region having first andsecond electrodes on opposite sides of the intrinsic region to detectphoton-generated caters formed in the intrinsic region and output thephotocurrent.
 11. The system of claim 10, wherein each detectorwaveguide includes a core, a cladding, and a lower surface and anintrinsic region disposed underneath the lower surface and surrounded bythe first and the second electrodes, wherein the core is longitudinallyaligned along the intrinsic region and optically coupled to theintrinsic region.
 12. The system of claim 10, wherein the couplingwaveguide is a single mode waveguide.
 13. The system of claim 10,wherein the system further includes a time-division multiplexer toprovide a multiplexed electrical signal to the optical transmitter. 14.The system of claim 10, wherein the system further includes atime-division demultiplexer to receive an electrical signal from theoptical receiver.
 15. The system of claim 10, wherein the opticaltransmitter, coupling waveguide, and the optical receiver are adapted asan on-chip communication system.
 16. A system comprising: an inputdevice that generates an optical signal; an optical edge tree comprisinga main waveguide optically coupled to the input device and a pluralityof equal-length waveguide branches extending from the main waveguide; awaveguide photodetector coupled to each waveguide branch, wherein eachwaveguide photodetector coupled to a corresponding waveguide branchincludes: a multiple mode interference (MMI) cavity having an input endand an output end; an input waveguide optically coupled to the MMIcavity at the input end, the input waveguide optically coupled to thecorresponding waveguide branch; an array of detector waveguidesoptically coupled to the MMI cavity at the output end and each opticallycoupled to an intrinsic region having first and second electrodes onopposite sides of the intrinsic region to detect photon-generatedcarriers formed in the intrinsic region and output a photocurrent; and aplurality of electronic edge trees comprising equal-length conductivebranches, with each conductive branch coupled to one of the waveguidephotodetectors to receive the photocurrent.
 17. The system of claim 16,wherein each detector waveguide includes a core, a cladding, and a lowersurface and an intrinsic region disposed underneath the lower surfaceand surrounded by the first and the second electrodes, wherein the coreis longitudinally aligned along the intrinsic region and opticallycoupled to the intrinsic region.
 18. The system of claim 16, whereineach electronic edge tree branch is connected to an output device. 19.The system of claim 16, wherein the input device includes a laser diodeor a vertical cavity surface emitting laser.
 20. The system of claim 16,wherein the input device is an optical clocking signal source toconfigure the system as an optoelectronic clocking source system.